Why use correlation coefficient?

Answer 1

It tells you if there is a relationship between two variables in a set of data, and if so, what kind of relationship.

The correlation coefficient, r, can be between -1 and 1.

If r = 1, there is exact direct variation (when x goes up, y goes up; the relationship is represented by a line with a slope of 1)

If r = -1, there is exact inverse variation (when x goes up, y goes down; the relationship is represented by a line with a slope of -1)

If r = -1, the variables aren't related.

If r = some other number between -1 and 1, the relationship is correspondingly stronger or weaker.